unsafe `set_len` breaks fragmentation rules

Assume we are using a `Doubling` strategy and currently we have 2 fragments with capacities of 4 an 8 respectively.
If the user calls `set_len(3)` while the vector is on this state, we will have:
* fragment 1 with len=3 and capacity=4
* fragment 2 with len=0 and capacity=8

This is an invalid fragment state. We must drop the second fragment.